The present invention relates to producing electrically conductive films cast from pH adjusted aqueous dispersions comprising electrically conducting polymers. In particular, the present invention is directed to resistivity stable films of thienothiophene polymers synthesized in the presence of at least on colloid forming polymeric acid.
Electrically conducting polymers have been used in a variety of organic electronic devices, including in the development of electroluminescent (EL) devices for use in light emissive displays. With respect to EL devices, such as organic light emitting diodes (OLEDs) containing conducting polymers, such devices generally have the following configuration:                anode/hole injection layer/EL layer/cathode        
The anode is typically any material that has the ability to inject holes into the otherwise filled π-band of the semiconducting material used in the EL layer, such as, for example, indium/tin oxide (ITO). The anode is optionally supported on a glass or plastic substrate. The EL layer is typically semiconducting, conjugated organic material, including a conjugated semiconducting polymer such as poly(paraphenylenevinylene), polyfluorene, spiropolyfluorene or other EL polymer material, a small molecule fluorescent dye such as 8-hydroxquinoline aluminum (Alq3), a small molecule phosphorescent dye such as fac tris(2-phenylpyridine) Iridium (III), a dendrimer, a conjugated polymer grafted with phosphorescent dye, a blend that contains the above-mentioned materials, and combinations. The EL layer can also be inorganic quantum dots or blends of semiconducting organic material with inorganic quantum dots. The cathode is typically any material (such as, e.g., Ca or Ba) that has the ability to inject electrons into the otherwise empty π*-band of the semiconducting organic material in the EL layer.
The hole injection layer (HIL) is typically a conducting polymer and facilitates the injection of holes from the anode into the semiconducting organic material in the EL layer. The hole injection layer can also be called a hole transport layer, hole injection/transport layer, or anode buffer layer, or may be characterized as part of a bilayer anode. Typical conducting polymers employed as hole injection layer include polyaniline and polydioxythiophenes such as poly(3,4-ethylenedioxythiophene) (PEDOT). These materials can be prepared by polymerizing aniline or dioxythiophene monomers in aqueous solution in the presence of a water soluble polymeric acid, such as poly(styrenesulfonic acid) (PSSA), as described in, for example, U.S. Pat. No. 5,300,575 entitled “Polythiophene dispersions, their production and their use”; hereby incorporated by reference in its entirety. A well known PEDOT/PSSA material is Baytron®-P, commercially available from H. C. Starck, GmbH (Leverkusen, Germany).
Electrically conducting polymers have also been used in photovoltaic devices, which convert radiation energy into electrical energy. Such devices generally have the following configuration:                positive electrode/hole extraction layer/light harvesting layer(s)/negative electrode        
The positive electrode and negative electrode can be selected from materials used for the anode and cathode of EL devices mentioned above. The hole extraction layer is typically a conducting polymer that facilitates the extraction of holes from the light harvesting layers for collection at the positive electrode. The light harvesting layer or layers typically consists of organic or inorganic semiconductors that can absorb light radiation and generate separated charges at an interface.
Aqueous electrically conductive polymer dispersions synthesized with water soluble polymeric sulfonic acids have undesirable low pH levels. The low pH can contribute to decreased stress life of an EL device containing such a hole injection layer, and contribute to corrosion within the device. Accordingly, there is a need in this art for compositions and hole injection layer prepared therefrom having improved properties.
Electrically conducting polymers also have utility as electrodes for electronic devices, such as thin film field effect transistors. In such transistors, an organic semiconducting film is present between source and drain electrodes. To be useful for the electrode application, the conducting polymers and the liquids for dispersing or dissolving the conducting polymers have to be compatible with the semiconducting polymers and the solvents for the semiconducting polymers to avoid re-dissolution of either conducting polymers or semiconducting polymers. The electrical conductivity of the electrodes fabricated from the conducting polymers should be greater than 10 S/cm (where S is a reciprocal ohm). However, the electrically conducting polythiophenes made with a polymeric acid typically provide conductivity in the range of about 10−3 S/cm or lower. In order to enhance conductivity, conductive additives may be added to the polymer. However, the presence of such additives can deleteriously affect the processability of the electrically conducting polythiophene. Accordingly, there is a need in this art for improved conducting polymers with good processability and increased conductivity.
Due to the limited lifetime of double or bilayer devices, more complicated device structures have been introduced to improve the device performance, especially lifetime. For example, a thin layer of a hole transporting and electron blocking material, which is known as an “interlayer”, has been shown to be effective in improving device performance and lifetime. Cambridge Display Technology reported enhanced lifetime with interlayer at OLEDs 2004 conference [David Fyfe, “Advances in P-OLED Technology—Overcoming the Hurdles Fast”, OLEDs 2004, San Diego, Calif. from Nov. 15 to 17, 2004; hereby incorporated by reference in its entirety]. So et al. reported 2× enhancement in efficiency and 7× enhancement in lifetime by inserting a crosslinkable hole transporting layer (XL-HTL) between the PEDOT:PSSA hole injection layer and a green polyfluorene light emitting layer. [Wencheng Su, Dmitry Poplavskyy, Franky So, Howard Clearfield, Dean Welsh, and Weishi Wu, “Trilayer Polymer OLED Devices for Passive Matrix Applications”, SID 05 Digest, Page 1871-1873; hereby incorporated by reference in its entirety].
Although these trilayer devices provide improved device performance and lifetime, the additional interlayer increases TACT time and/or manufacturing capital cost, and might decrease device yields. There is a need in this art for a double layer device with improved device performance and lifetime.
In our pending patent applications (U.S. patent application Ser. No. 11/240,573), we found that the lifetime performance of OLEDs can be improved significantly using conductive polymers based on PTT and fluorinated polymeric acid as the hole injection layer. We also found that the conductivity (resistivity) of films cast from aqueous dispersions of conductive polymers based on PTT and fluorinated polymeric acid changes significantly upon annealing at elevated temperatures. This is advantageous if the targeted application can accept the conductivity change. However, in some applications, the resistivity change is not ideal. In the latter case, there is a need to suppress the resistivity change during the process, especially during annealing or baking of devices after different layers, such as the hole injection layer and the light emitting layer, have been coated.
Therefore what is needed is a process for fabrication of electrically conductive polymers and electrically conductive polymers produced having controlled electrical resistivity.